, 1995; Geiger et al., 1999; Weissenmayer et al., 2002; Gao et al., 2004). Challenging of some bacteria with low pH conditions can cause the modification of already existing membrane Target Selective Inhibitor Library molecular weight lipids, such as the formation of lysyl-phosphatidylglycerol from phosphatidylglycerol

or the hydroxylation of OLs (Rojas-Jiménez et al., 2005; Sohlenkamp et al., 2007; González-Silva et al., 2011; Vences-Guzmán et al., 2011). The capacity to form OLs is apparently widely distributed in eubacteria, but so far, OLs have not been detected in archaea and eukaryotes (López-Lara et al., 2003; Geiger et al., 2010). They contain a 3-hydroxy fatty acyl group that is attached in amide linkage to the α-amino group of ornithine. A second fatty acyl group, the so-called

piggy-back fatty acid, is ester-linked to the 3-hydroxy position of the first fatty acid (Knoche & Shively, 1972; Geiger et al., 1999). In some bacteria, OLs can be modified by hydroxylation in one or more positions. In recent years, several genes coding for OL hydroxylases have been identified. OLs can be hydroxylated in the ester-linked fatty acid, the amide-linked fatty acid, and the ornithine moiety (Rojas-Jiménez et al., 2005; González-Silva et al., 2011; Vences-Guzmán et al., 2011). In Gluconobacter cerinus, OLs hydroxylated in the C-2 position of the ester-linked fatty acid can be modified with a taurine residue that is amide-linked to the α-carboxy group of ornithine. This tauro-OL is also called cerilipin after the bacterial species from which it was isolated (Tahara et al., b). Although OLs are present in both membranes of Gram-negative bacteria, they are more abundant in the outer click here membrane (Dees

& Shively, 1982; Palacios-Chaves et al., 2011; Vences-Guzmán et al., 2011). Structurally similar lipids in which other amino acids are present instead of ornithine have been described. A lysine lipid has been described in an Agrobacterium tumefaciens strain (Tahara et al., b), glycine lipids were detected in Cytophaga johnsonae and Cyclobacterium marinus (Kawazoe et al., 1991; Batrakov et al., 1999), glutamine pheromone lipids were described in Rhodobacter sphaeroides (Zhang et al., 2009, 2011), and serineglycine lipids (SGLs) were isolated from the opportunistic pathogen Flavobacterium meningosepticum (Kawai et al., 1988; Shiozaki et al., b). The biosynthesis of the unmodified OL (sometimes also called S1 (Rojas-Jiménez et al., 2005)) occurs in two steps. The genes coding for the acyltransferase activities OlsB and OlsA required for OL biosynthesis were first discovered in the α-proteobacterium Sinorhizobium meliloti (Weissenmayer et al., 2002; Gao et al., 2004). In the first step, the N-acyltransferase OlsB is responsible for the transfer of a 3-hydroxy fatty acyl group from 3-hydroxy fatty acyl-acyl carrier protein (ACP) to the α-amino group of ornithine, thereby forming lyso-ornithine lipid (LOL) (Gao et al., 2004).